Polycarbonate resins are well known, commercially available materials possessing excellent physical and chemical properties which are useful in a wide variety of applications. Such polymers or resins, since they are less dense and more breakage resistant than glass, have been especially useful as substitutes for glass, as for example, in the manufacture of tail lights and stop light lenses, protective shields for fluorescent street lights, safety shields in inspection windows, windshields, windows, and the like. However, these resins have relatively low mar and chemical solvent resistance.
In order to overcome this relatively low mar and chemical solvent resistance, polycarbonate articles have been coated with various organic and inorganic protective layers which increase the mar resistance of said polycarbonate articles. One type of inorganic protective layer is comprised of glass which has been vapor deposited onto the polycarbonate substrate. Thus, for example, French Pat. No. 1,520,125 and the corresponding British Pat. No. 1,144,099 teach that the surfaces of polycarbonates can be improved, especially rendered more scratch resistant, by vapor depositing an SiO.sub.2 layer of at least 1.sub.u thickness onto the polycarbonate. This vapor deposition is accomplished by evaporating SiO.sub.2 with an electron beam evaporator source in a high vacuum in the presence of oxygen while regularly moving the polycarbonate article to be coated in the vapor jet and/or the electron beam evaporator source in such a manner that at least 50 successive layers are evaporated onto the surface of the polycarbonate article.
However, this electron beam vapor deposition method suffers from several disadvantages. One of these is due to the electron beam source being, in effect, a point source of SiOx molecules. Thus, the area of the substrate immediately above the electron beam receives a thicker coating of glass then do the cross peripheral to the point of impingement of the SiOx molecules. Consequently, to provide a glass coating of uniform thickness a plurality of electron beam sources need be used or, alternately, the single electron beam source must be rapidly oscillated along the width of the substrate. Thus, it is generally difficult to obtain a glass layer of uniform thickness on large substrates utilizing an electron beam evaporator device.
Furthermore, the articles thus prepared have been found to be not entirely satisfactory since, under high stress or temperature changes, the SiO.sub.2 protective layer tends to crack and/or separate from the polycarbonate article. In order to overcome this cracking and separation of the protective silicate glass layer, various modifications of the basic vapor deposition process have been proposed. Thus, British Pat. No. 1,313,866 teaches a polycarbonate having a vapor deposited protective layer consisting of SiO.sub.2 and 5 to 10% zirconium oxide. Similarly, U.S. Pat. No. 3,645,779 teaches a synthetic polymer provided with a hard, abrasion-resistant surface free of fine hairline cracks by vapor depositing under vacuum onto the surface of said polymer a B.sub.2 O.sub.3 --SiO.sub.2 glass containing less than 5 percent by weight of Na.sub.2 O. U.S. Pat. No. 3,713,869 teaches disposing an intermediate layer between the plastic substrate and the vapor deposited glass layer for the purpose of improving the adherence of said glass layer. This intermediate layer comprises a polymerization layer which is formed by subjecting low-molecular organic vapors to a glow discharge operation and depositing the polymerization products on the substrate. The organic vapors are provided by organic compositions such as acetylene, xylol, and those compounds which contain Si, preferably in an SiO bond, such as silicate acid methyl or silicic acid ethylester, and low boiling siloxanes.
By the method of the instant invention it is possible to obtain vapor deposited glass coatings of uniform thickness and good adhesion on large substrates. By utilizing the method of the present invention it is possible to produce a polycarbonate article having a protective top layer of vapor deposited glass which is free of cracks, is of a uniform thickness and is tenaciously and durably adhered to the polycarbonate substrate. The advantages of the instant invention include the fact that the glass used to form the protective layer need not be of any special composition, as is the case in the afore-mentioned U.S. Pat. No. 3,645,779 and British Pat. No. 1,313,866; that the intermediate primer layer aids in providing solvent protection for the polycarbonate substrate; and that the article produced by the instant method, since the polycarbonate substrate is already coated with the intermediate layer before it is exposed to the vapor deposition process, is relatively easy and simple to manufacture, i.e., the conditions existent during vapor deposition can be more variable than if there were no intermediate layer and if radio frequency induction evaporation were not used.